This invention relates to aerobic sewage treatment systems. More particularly, this invention relates to self-contained, closed system, aerobic sewage treatment systems having an interconnected series of aerated tanks which progressively purify the waste to essentially organic-free water.
The process of biochemical aerobic digestion of organic waste such as raw sewage is well known. Basically the process consists of aerating raw sewage in an aeration tank. The aeration promotes the growth of microorganisms which feed on and hence digest the raw sewage. Digestion converts the raw sewage primarily into carbon dioxide and water along with sludge. The resultant water-sludge mixture is drawn off into a settling tank. After settling, the sludge is decanted. A small portion of the sludge, as a supernatant, is returned to the aeration tank to innoculate the incoming raw sewage with microorganisms and stimulate rapid synthesis of additional microorganisms. The balance of the supernatant sludge-supernatant is discharged from the system, usually after chlorination.
Problems associated with such sewage treatment systems include the buildup of sludge in the settling tank and the discharge of impure effluent into the environment.
As claimed in U.S. Pat. No. 3,990,967 (Hargraves), substantially complete digestion of organic waste by biochemical action without the production of excessive sludge can be obtained by a progressive biochemical treatment carried out in a series of aerated sewage treatment tanks. Bascially the treatment involves the retention of the large organic solids, both floating and non-floating, in a first treatment or main digestion tank. After vigorous aeration and substantial digestion, the effluent is passed sequentially through a series of secondary treatment tanks where further settling and biochemical purification takes place. Eventually, substantially waste-free liquid is supposedly obtained in the final tank.
Since biochemical digestion is enhanced by vigorous aeration and turbulent movement of the solids and liquid, the Hargraves patent teaches the use of an air diffusor disposed at the bottom of each tank. Air is forced out of the air diffusor so as to cause a rolling motion of the liquid-solid mixture. The rolling motion is claimed to have the favorable effect of: (1) supplying oxygen directly to the microorganisms; (2) moving the microorganisms to the surface of the liquid-solid mixture within the tank to thereby expose the microorganisms to the air above the mixture; and (3) breaking up the solids into smaller and smaller particles as they roll around in each tank.
Unfortunately, in actual practice the Hargraves effects are not attained to the degree necessary for adequate digestion. This inability is most acute in the main digestion tank where break up and aeration of the solids is essential for the effective operation of the entire system. Inadequate digestion of the solids in the main digestive tank will unduly pollute the secondary tanks. The process of having progressively purer secondary stages is, therefore, defeated.
The Hargraves patent attempts to adequately break up and aerate the solids in the main digestion tank by locating the air diffusor below a screen (see FIG. 2). Air flowing out of the diffusor aerates and agitates the solids as they settle on the screen. In practice, clogging of the screen and inadequate aeration of the settled solids not directly above the diffusor occurs.
The Hargraves patent also incorporates an air lift within the main digestion tank (see FIG. 10). Here, the solids are air lifted up a bent conduit and are then forcibly discharged against a wall member to break up the solids. In practice, the air bubbles are unable to air lift large solids up the conduit. Such large solids then clog the conduit.
Another problem, as noted earlier, is retention of the larger solids in the main digestion tank. Only smaller solids should be passed on to subsequent digestion tanks. To insure such, the Hargraves patent (FIG. 2) shows a conduit disposed within each digestion tank. The submerged opening of the conduit is covered with a filter. The filter prevents suspended large solids from flowing up the conduit and into the subsequent digestion tank. Unfortunately as can be expected with all filters, the clogging which eventually occurs necessitates the removal and cleaning of the filter. This problem is most acute in the main digestion tank where the larger solids are encountered.
The Hargraves patent also uses quiescent chambers in each tank to prevent larger solids from flowing into subsequent stages (see FIGS. 8 and 9). A horizontal conduit connected to a vertical conduit are disposed within each digestion tank. The liquid in the vertical conduit is substantially stagnant, thereby facilitating the settling of larger solids contained therein to the bottom of the digestion tank where they are then drawn away by the rolling motion of the liquid in that tank. Such a quiescent chamber has several drawbacks including: (1) the unstreamlined configuration of the quiescent chamber disrupts the rolling motion of the liquid contained in that digestion tank; (2) the submerged opening of the quiescent chamber is of such large diameter that the rolling motion of the liquid in that digestion tank agitates the liquid contained in the quiescent chamber; and (3) undue tilting of the digestion tank causes flooding of the subsequent stage.
Therefore an object of this invention is to provide a sewage treatment system which uniquely avoids sludge buildup.
Another object is to provide a self-contained sewage treatment system thereby precluding the need to discharge effluent into the environment.
Still another object is to provide a means to progressively biochemically treat raw sewage.
A further object is to provide a novel means to vigorously aerate the solids suspended in the liquid of each digestion tank in a sewage treatment system.
A still further object of this invention is to provide means to insure that only the smaller particles suspended in the liquid in a preceding digestion tank are passed on to the subsequent digestion tank.
Another object is to provide means to insure that the raw sewage is vigorously aerated in the main digestion tank without requiring screens or filters which may become clogged.
Another object is to provide means to break-up the solids contained in the main digestion tank and other secondary digestion tanks.
Another object is to provide streamlined quiescent chambers which do not interfere with the rolling motion of the liquid in each digestion tank.
Another object is to provide means to insure that the rolling motion of the liquid in each digestion tank does not agitate the stagnant liquid contained in the quiescent chamber.
Another object is to provide means to help prevent flooding of a subsequent digestion stage should the sewage treatment system be utilized in a rocking or otherwise unstable environment.
Another object is to provide means for the sewage digestive system to be incorporated into a portable, self-contained toilet facility which may be used in a large variety of situations such as on a boat, plane, mobile home, etc.
Other objects of the invention will in part be obvious and will in part appear hereinafter.